Romans 1:9

For God is my witness, whom I serve
These words are an appeal to God, and carry in them the nature and form of an oath; the reason of the apostle's using it was, because he was personally unknown to the Romans, and they to him, and so might doubt of his affectionate regard unto them; and therefore for the confirmation thereof he uses it: this was a case which was only known to God and himself, and hence he appeals to him for the truth of it. The object of his oath or appeal, or by which he speaks, is not himself, or anything that belonged to him, nor any creature in heaven or on earth, but God; who in a solemn oath is only to be appealed to and sworn by: he describes him as the God "whom [he] served", to distinguish him from all false gods, and to show that he that takes an oath, should be one that fears and serves the Lord; what he served him in was not the law, but

the Gospel of his Son;
Jesus Christ, who is the author, minister, and subject matter of it: he served him in it, by preaching, spreading, and defending it. This is a service, and a very laborious one, and makes for the honour and glory of God. The manner in which he served him was, as he says,

with my Spirit;
either with the Spirit of God, which was given to him; or in a spiritual manner, in opposition to the carnal worship of the Jews; internally, in opposition to bodily exercise only, and voluntarily, with his whole heart, soul, and spirit. The matter or substance of his appeal or oath was,

that without ceasing I make mention of you always in my prayers;
whence may be observed, that prayer to God ought to be constant; and that we should be concerned for others as well as for ourselves; all the saints should share therein.